Remote monitoring of fluid flow provides utility across a wide range of industrial and healthcare applications. In medical diagnosis, treatment and monitoring, there is frequent need to measure blood flow in order to monitor tissue perfusion, metabolism and general health. This need stems from the biological requirement for transport of oxygen and nutrients to tissues and the removal of waste products (M. E. Raichle and M. A. Mintun, “Brain work and brain imaging”, Annu Rev Neurosci 2006; 29:449-476).
Existing methods for measuring biological flow include laser Doppler measures of reflected energy changes (as disclosed for example in U.S. Pat. No. 41,096,47), variation in impedance on the scalp surface (e.g. US 2008/0200787), or the use of invasive flowmeters implanted directly into the tissue (e.g. U.S. Pat. No. 5,579,774).
Various techniques for non-invasive measurements of blood related parameters have been developed. Such techniques include frequency-domain spectroscopy, and photoacoustic spectroscopy [D M Hueber et al Phys. Med. Biol. 46 (2001) 41-62].
A productive approach to non-invasive monitoring in healthcare settings is the use of ultrasound tagged light. (U.S. Pat. No. 7,541,602, WO 06/097910, WO 05/025399, all assigned to the assignee of the present application). In this method, the intersection of electromagnetic and acoustic signals has been used to monitor tissue optical properties.